| Lithium-sulfur batteries(Li-S)have been considerated as one of the most promising next-generation power sources,owing to their high theoretical specific capacity and high energy density,low cost,and abundant resources.However,the sulfur cathode must be paired with metallic lithium anode,suffering from safety issues,so the researchers start to pay their attention to Li2S cathode,which can be paired with lithium-free anodes to avoid safety issues.But Li2S has similar obstacles to pure sulfur cathode,namely,poor electrical conductivity and solubility of polysulfides into the electrolytes.Another issue for Li2S is that the phase nucleation from Li2S to polysulfides needs a high charging cutoff voltage to overcome the charge barrier of micrometer-sized Li2S.To resolve the abovementioned problems,this paper focus on synthesizing nanostructure Li2S particles to reduce the charge barriers and double modification with constructing conductive network and carbon shell to enhance the electrochemical performance of Li2S cathodes.The main works can be summarized as follows:(1).Conductive three-dimensional reduced graphene oxide(3D-rGO)with high specific surface area is consturcted by hydrothermal method,and 3D-rGO-Li2S@C composite is synthesized through solution-evaporation methode and PVP carbonization.The 3D-rGO constructs three-dimentional porous conductive network that not only can support a content of up to 75 wt%of lithium sulfide,but also can provide fast electron transport path.Polyvinylpyrrolidone(PVP)not only can prevent the Li2S from agglomeration during evaporation,but also form a carbon coating in order to improve the electricity and prohibit polysulfides from "shuttling".The 3D-rGO-Li2S@C cathode delivers an intial discharge capacity of 856 mAh g-1 at 0.1C,and the capacity remains 563 mAh g-1 after 100 cycles.(2).The vertical graphene(VG)arrays are synthesized by plasma enhanced chemical vapor deposition(PECVD),and a free-standing VG/Li2S-C cathode is fabricated through a facile liquid-solution/evaporation method in combination with CVD.VG arrays with high conducivity and crystallinity construct a conductive network for loading Li2S nanoparticles and fast electron transport path.And the carbon shell not only improves the conductivity of Li2S,but also acts as a durable protective shield against polysulfide shuttle during cycling.VG/Li2S-C cathode delivers a discharge capacity of 656.2 mAh g-1 after 100 cycles at 0.1 C,and the capacity retention is 73.7%,owing to the double modification of VG and carbon shell.(3)An integrated CF-CB-Li2S@C cathode with high loading Li2S is prepared through a facile liquid solution-evaporation plus CVD technology.The CF possesses a hierarchical architecture with abundant macroporous channels and provides enough reaction sites to load and stabilize Li2S with an areal loading of 7 mg cm-2.And the protective carbon coating not only improves the electronic conductivity of Li2S,but also prohibits the polysulfide dissolution;thus,superior electrochemical performances with an intial discharge capcity of 943.7 mAh g-1 and an areal capacity of 6.60 mAh cm-2 at 0.1 are achieved.(4)Three-dimensional reduced graphene oxide supported Li2S/carbon composite(3D-rGO-Li2S/C)is synthesized through an in-situ facile route by using the lithium sulfate and glucose as the raw material.The 3D-rGO constructs three-dimentional porous conductive network that can provide enough reaction sites for lithium sulfide and fast electron transport path.Carbon formed by carbonization of glucose not only takes part in in-situ reaction,but also improves the conductivity of Li2S and inhibits dissolution and shuttling of polysulfides.As a result,3D-rGO-Li2S/C cathode shows an intial discharge capacity of 819 mAh g-1 at 0.1 C and a dischaege capacity of 469 mAh g-1 can be remained after 100 cycles with a Coulomic efficiency almost 100%.(5)The free-standing carbon paper supported Li2S@C(P-Li2S@C)integrated cathode is synthesized through pyrolysis of from low-cost Li2SO4 and chitosan on napkin paper and outer CVD technology,as napkin paper has a good absorption for Li2SO4 and chitosan solution.Chitoson can prevent the Li2SO4 from agglomeration in order to pepare nanostructure Li2S as well as taking part in the in-situ reaction.The CVD-coated carbon shell effectively increases the conductivity of Li2S and suppresses the dissolution of polysulfides,improving the electrochemical performance of cathode.The P-Li2S@C cathode delivers an intial discharge capacity of 820 mAh g-1 at 0.1 C,and the capacity remains 430 mAh g-1 after 100 cycles. |
